Tiny Black Specks on Your Deck: Frass, Fungus, or Droppings?

Tiny black specks on a deck usually turn out to be caterpillar frass raining down from trees overhead, artillery fungus spores that glue themselves to surfaces, spider droppings, or mouse droppings. The quickest test is whether the specks wipe away: frass and droppings brush off, while artillery fungus dots are stuck fast. Where the specks appear — under a tree canopy, near mulch, or along the house wall — points to the source.

Most likely causes

  • Caterpillar frass — dry pellet dust falling from trees directly overhead
  • Artillery fungus — tar-like black dots glued on, near mulch beds; not droppings at all
  • Spider droppings — small paint-like splatters below webs and light fixtures
  • Mouse droppings — 1/4-inch pointed pellets along the house wall or railing base
  • Flea dirt — pepper-like specks near pet bedding that smear reddish when wet

Compare the possible causes

Possible cause Key signs When it happens How likely
Caterpillar frass Dry, pepper-to-BB-size pellets scattered evenly under the tree canopy, with a faintly ridged, segmented look under magnification Late spring through summer, during heavy caterpillar feeding; can appear fresh every morning Very common
Artillery fungus Glossy black dots about 1/16 inch across that are cemented to the surface and will not wipe off, usually within 10–20 feet of a mulch bed Cool, damp weather in spring and fall Common
Spider droppings Small drips and splats like flicked paint, often dark with a whitish component, concentrated beneath a web, corner, or porch light Summer through fall, when spiders are large and active around lights Common
Mouse droppings Firm pellets about 1/4 inch long with pointed ends, following the edge of the house wall, a railing base, or the gap under a door Year-round, appearing overnight; traffic increases in fall Common
Flea dirt Fine pepper-like specks concentrated where a dog or cat sleeps, which smear rusty red when dabbed with a wet paper towel Warm months, peaking mid to late summer Less common

Visual clues to check

  • Try to wipe a speck away: brushes off easily suggests frass or droppings; glued on tight means artillery fungus
  • Look straight up: a tree canopy overhead points to caterpillar frass; a web or light fixture points to spider droppings
  • Check the pattern: even scatter matching the canopy is frass, lines along walls are mice, splatter under one spot is spiders
  • Do the wet paper towel test near pet areas: a rusty red smear means flea dirt
  • Measure the specks: pinhead-size dust is frass or flea dirt; distinct 1/4-inch pellets with pointed ends are mouse droppings
  • Wipe a test section clean in the evening and check at mid-morning — overnight specks suggest mice or spiders, daytime accumulation under a tree suggests caterpillars

The causes in detail

Caterpillar frass

When oaks, maples, and other shade trees carry a big caterpillar population, their droppings — called frass — fall like slow rain and collect on everything below. Frass pellets are dry, crumbly, and uniform, and the deposit matches the outline of the canopy overhead. A heavy, persistent frass fall is worth a look upward: it can signal a defoliating outbreak such as spongy moth or fall webworm chewing through the tree.

Artillery fungus

Artillery fungus grows in wood mulch and shoots its sticky black spore packets toward light-colored surfaces — deck boards, siding, cars — with surprising force and range. The dots look exactly like specks of tar and bond so firmly that scraping often leaves a stain. It isn't an animal and isn't harmful, just stubborn and ugly. If your specks pass the fingernail test (stuck fast rather than brushing away) and there's mulch nearby, this is your answer.

Spider droppings

Spider droppings aren't pellets at all — they're liquid, so they land as tiny pin-head splatters and thin drips on railings and boards directly below where the spider sits. Porch lights are hotspots because they concentrate the flying insects spiders eat. Look up and you'll almost always find the web or the spider tucked into a corner within a foot or two of the marks.

Mouse droppings

Mouse droppings are bigger than frass and follow travel routes rather than falling in a canopy-shaped scatter. You'll find them in lines along the deck's junction with the house, under a grill cabinet, or near stored seed and pet food. Fresh pellets are dark and shiny; if new ones keep appearing along the same edge, mice are patrolling the deck nightly and may be probing the house for entry.

Flea dirt

Flea dirt is flea droppings — digested blood — and it accumulates where pets rest, not across the whole deck. The wet-towel test is definitive: place a few specks on a damp white paper towel, and flea dirt dissolves into a reddish-brown halo. Finding it means the pet, its bedding, and the resting area all need attention for fleas.

When to worry

  • Frass falling heavily every day while the tree above shows thinning leaves — a defoliating caterpillar outbreak
  • Mouse droppings that reappear along the house wall within a day of cleaning, especially near a door gap or vent
  • Flea dirt on the deck plus a scratching pet — the flea population is established, not incidental
  • Specks accumulating on outdoor furniture where small children eat and play

What to do now

  1. Run the wipe test and the wet-towel test first — two minutes of checking beats guessing
  2. For frass, simply hose off the deck; treat the tree only if defoliation is significant, ideally after an arborist confirms the culprit
  3. For artillery fungus, scrub new dots promptly (they bond harder with time) and consider switching nearby beds from wood mulch to stone or pine straw
  4. For mouse droppings, wear gloves, wet the pellets with disinfectant before wiping, remove seed and pet food, and seal gaps along the house wall
  5. For flea dirt, wash pet bedding in hot water and talk to your vet about flea prevention for the pet
  6. If mouse sign persists after cleanup and exclusion, bring in a licensed pest professional

What not to do

  • Don't sweep or leaf-blow dry mouse droppings — dampen and wipe them instead
  • Don't touch any droppings bare-handed, even specks this small
  • Don't power-wash artillery fungus expecting success — it usually just removes the finish around the dots
  • Don't spray the deck with insecticide before knowing whether the source is a tree, a spider, or a rodent — most of the time it solves nothing

Think you know the suspect?

These animals commonly cause this clue — see their full sign profiles:

Frequently asked questions

What are the black dots on my deck that won't come off?

Dots that are glued down and resist scrubbing are almost certainly artillery fungus, a mulch-dwelling fungus that fires sticky black spore masses at light surfaces. It's not an animal and not a health hazard, but the spots bond permanently if left. Scrub new ones quickly and consider replacing nearby wood mulch with stone.

How do I know if the specks are caterpillar frass or mouse droppings?

Pattern and position. Frass falls from above, so it scatters evenly across the deck in the shape of the canopy overhead and reappears during the day. Mouse droppings appear overnight in lines along walls, railings, and sheltered edges, and each pellet is a distinct 1/4-inch grain with pointed ends.

Is caterpillar frass harmful to people or pets?

Not meaningfully — it's digested leaf material. It's unsightly and can stain light surfaces if it gets wet, so hose it off rather than letting it sit. The real question is whether the tree above is being seriously defoliated, which is an arborist question, not a health one.

Why do the specks only show up under my porch light?

That points to spiders. Lights draw night-flying insects, spiders set up webs to catch them, and their liquid droppings splatter the surface directly below. Relocating or dimming the light, or switching to a yellow bug bulb, reduces the insects, the spiders, and the mess.

Related yard clues

Animal Droppings Black Droppings on Your Patio: Rat, Mouse, Bat, or Bird? Black droppings on a patio most often come from mice, rats, or bats, with birds and squirrels as runners-up. Mouse droppings are about 1/4 inch with pointed ends, rat droppings run 1/2 to 3/4 inch with blunt ends, and bat droppings look mouse-sized but crumble into shiny insect fragments. Where the droppings sit — piled under the eaves versus scattered along a wall — narrows it down fast. Read the guide → Animal Droppings Droppings in the Garage: Mouse, Rat, Bat, or Squirrel? Droppings in a garage are most often from mice, with rats, bats, and squirrels as the other usual suspects. Mouse droppings are 1/4-inch pellets with pointed ends scattered along walls and shelves; rat droppings are two to three times larger with blunt ends; bat droppings pile up in one spot beneath a roost near the ceiling or wall top. Before you identify anything, know the cleanup rule: never sweep or vacuum rodent droppings dry. Read the guide → Animal Droppings White Droppings on Your Fence: Which Bird (or Lizard) Was It? White droppings on a fence are almost always from birds — the white part is uric acid, which is how birds excrete waste instead of urine. The size and pattern tell you whether it's songbirds using the fence as a perch, larger birds like doves or hawks, or, in the southern U.S., geckos and anoles, whose small dark droppings carry a distinctive white tip. Light deposits are harmless; large accumulations deserve careful cleanup. Read the guide →

Still not sure? Compare with these clues

Mushrooms & Growths Slime Mold in Mulch: The Gross-Looking Blob That's Harmless A foamy yellow, tan, or orange blob spreading across your mulch is almost certainly a slime mold — most often the species nicknamed 'dog vomit slime mold.' Despite the revolting look, it's completely harmless to plants, pets, and people, it usually rides in on fresh hardwood mulch, and it needs no treatment: rake it out, hose it apart, or just let it dry up on its own. Read the guide → Mushrooms & Growths Orange Fungus in Mulch: Stinkhorns, Slime, and Siding Spots Orange growths in mulch are almost always fungi feeding on the wood itself — most often stinkhorns (the smelly orange fingers), orange peel fungus, or a slime mold. They're harmless to plants and people who leave them alone. The one mulch fungus that costs money is artillery fungus, which shoots tar-like black dots onto siding and cars. Read the guide → Bugs & Eggs Green Caterpillars on Plants: Which One Is Eating Yours? The green caterpillar on your plants is most likely a cabbage worm if it's on broccoli or kale, a tomato hornworm if it's large and has a tail spike, or a sawfly larva if it's on roses. Which plant is being eaten narrows it down fast, and handpicking plus row covers handle most of them — no spraying required. Keep in mind that some green caterpillars grow up to be butterflies worth keeping around. Read the guide → Plant Damage Sticky Residue on Leaves: Honeydew and What's Making It A sticky, shiny film on leaves is honeydew — the sugary waste excreted by sap-sucking insects like aphids, soft scale, whiteflies, or mealybugs feeding somewhere above the sticky spot. Look up and check leaf undersides to find the source, and don't be surprised if ants are patrolling the plant: they farm these insects for the honeydew. Read the guide →